Centrifugal separator having a vortex generator

ABSTRACT

A centrifugal separator and a method for separation of particles from a gas stream is disclosed. The separator includes a frame, a gas inlet and a gas outlet. A centrifugal rotor is arranged to be rotatable in the frame around a rotational axis and includes a plurality of separation plates defining separation passages between the plates. A central gas chamber in the rotor communicates with a radially inner portion of the separation passages and the gas outlet. A space surrounding the rotor communicates with a radially outer portion of the separation passages and the gas inlet. A device is configured to bring the gas stream in rotation upstream of the rotor. The centrifugal rotor is configured such that the rotational flow of the gas mixture drives the rotation of the centrifugal rotor for separating particles from the same gas stream being conducted from the space surrounding the rotor, through the separation passages between the plates and towards the central gas chamber.

TECHNICAL FIELD

The present invention relates generally to a centrifugal separator and amethod for separation of particles from a gas stream.

BACKGROUND

WO 2010/090578 A1 discloses a centrifugal separator plant for separatingoil in form of particles and/or mist from a fossil gas mixture forobtaining a separated gas. The plant comprises a centrifugal separatorwith a stationary casing defining a separation space. The centrifugalseparator comprises an inlet for the gas mixture, a gas outlet for theseparated gas and an oil outlet for discharging separated oil. Aseparating member for separating the gas mixture comprises a pluralityof separating discs and is provided in the separation space. A drivemotor is connected to the separating member via a spindle and rotatesthe separating member about an axis of rotation.

SUMMARY

An object of the present invention is to simplify the construction andoperation of a centrifugal separator for separation of particles from agas stream, such as the type of centrifugal separator disclosed in thebackground art. Another object is to reduce the cost of this type ofcentrifugal separator. It is also an object to reduce the pressure dropover the centrifugal separator.

Thus the present invention relates to a centrifugal separator forseparation of particles from a gas stream. Particles are defined assolid and/or liquid particles, such as oil droplets or oil mist. Thecentrifugal separator comprises a frame, a gas inlet and a gas outlet. Acentrifugal rotor is arranged to be rotatable in the frame around arotational axis and comprises a plurality of separation plates, such asfrustoconical separation discs or axial plates, defining separationpassages between the plates. A central gas chamber is formed in therotor and communicates with a radially inner portion of the separationpassages and the gas outlet. A space surrounding the rotor communicateswith a radially outer portion of the separation passages and the gasinlet. The centrifugal separator further comprises a vortex generatorconfigured to bring the gas stream in rotation upstream of the rotor,and the centrifugal rotor is further configured such that the rotationalflow of the gas mixture drives the rotation of the centrifugal rotor forseparating particles from the same gas stream being conducted from thespace surrounding the rotor, through the separation passages between theplates and towards the central gas chamber.

Thus the centrifugal rotor is brought into rotation by the rotationalflow of the gas stream, whereby the centrifugal separator may beindependent on a separate drive motor to drive the rotor. Thereby theconstruction of the separator may be simplified, the cost may be reducedand the need for service and maintenance of the separator may bedecreased. Since the rotor is driven in rotation by the rotational flowof the gas, the rotational speed of the rotor is similar to therotational speed of the gas entering into the separation passages. Thisis particularly beneficial since it reduces the pressure drop over theseparator. Further, since the rotating gas stream is led from theradially outer portions of the separation passages and towards theradially inner portions of the separation passages, the gas stream isspun up thanks to the conservation of angular momentum. Thus thetransfer of the rotation from the gas to the rotor, such as by viscousforces, is particularly efficient. The centrifugal rotor may preferablybe configured such that the rotational flow of the gas mixture alonedrives the rotation of the centrifugal rotor for separating particlesfrom the same gas stream, whereby the centrifugal rotor is not driven bya motor connected to the rotor.

The vortex generator configured to bring the gas stream in rotation maypreferably be disengaged from the rotor, such that the rotor may rotateindependently of the vortex generator configured to bring the gas streamin rotation.

The vortex generator configured to bring the gas stream in rotation maybe stationary or at least non-rotating during operation of theapparatus. The vortex generator configured to bring the gas stream inrotation may comprise a gas deflecting ring connected to the frame,configured to bring the gas stream in rotation by deflecting the gasstream towards a tangential direction of the centrifugal rotor. The gasdeflecting ring may comprise at least one, preferably a plurality ofvanes inclined with respect to the axial direction of the centrifugalrotor and distributed around the rotational axis. The vanes maypreferably be arranged at a large radius of the centrifugal separatorwith respect to the rotational axis, such extending radially outside theseparation plates of the rotor or at a radial position close to orradially outside the radially outer portions of the separation passages.Thus a pre-separation of particles from the gas stream may be performedin the rotating gas in the space radially outside the separation plates.The inclination of the vanes with respect to the axial direction of thecentrifugal rotor may increase gradually along the extent of the vanesin the direction of the flow of the gas stream from the gas inlet to thegas outlet to provide smooth acceleration of the gas stream intorotation. Thus the rotational flow of the gas stream may be providedefficiently by a robust and uncomplicated construction which may befitted close to the rotor and provided in line with the centrifugalrotor and which makes use of the momentum of the gas stream to providethe rotational flow.

The inclination of the vanes with respect to the axial direction of thecentrifugal rotor may be adjusted during operation of the separator suchthat to control the rotational speed of the gas stream. Thus therotation of the centrifugal rotor may be controlled by adjusting theinclination of the vanes. In particular, the rotational speed of therotor may be limited by limiting the rotational speed of the gas streamin the vessel.

The gas inlet may be arranged at an angle to the rotational axis of thecentrifugal rotor, wherein the angle is within the range of 70-110degrees, preferably 80-100 degrees, more preferably 90 degrees, andwherein the vortex generator configured to bring the gas stream inrotation upstream of the rotor comprises an inlet gas deflecting memberwhich is arranged to deflect the gas stream from the gas inlet towards atangential direction of the centrifugal rotor. Thus the rotational flowof the gas stream may be provided while connecting the gas inlet at anangle close to 90 degrees to the vessel wherein the centrifugalseparator is arranged. Thereby the connection may be configured towithstand high pressure in the vessel, while the inlet gas deflectingmember causes the rotational flow of the gas stream.

The gas inlet may alternatively be arranged in line with the rotationalaxis of the centrifugal rotor, and the vortex generator configured tobring the gas stream in rotation upstream of the rotor may comprise aninlet gas deflecting member which is arranged to deflect the gas streamfrom a direction along the rotational axis towards a tangentialdirection of the centrifugal rotor.

The inlet gas deflecting member may be arranged upstream of the gasdeflecting ring, to provide a pre-separation of particles from the gasstream such that in the form of a cyclonic separator.

The position or inclination of the inlet gas deflecting member may beadjusted during operation of the separator such that to control therotational speed of the gas stream. Thus the rotation of the centrifugalrotor may be controlled by adjusting the inclination of the inlet gasdeflecting member.

The vortex generator configured to bring the gas stream in rotation maycomprise a rotating fan arranged upstream of the rotor.

The rotor may have a first and a second axial end portion, and the rotormay be rotatably supported in the frame by means of a first bearing atthe first axial end portion and a second bearing at the second axial endportion. Thus a more stable rotor construction may be achieved.

The frame may be configured to be mountable inside a vessel for guidingthe gas stream, and may comprise a first partition for dividing thevessel into a first section upstream of the first partition and a secondsection downstream of the first partition, wherein the gas inlet iscommunicating with first section, the gas outlet is communicating withsecond section, and wherein the centrifugal separator is configured suchthat the first and second sections communicate via the separationpassages of the rotor. A seal may be provided between the firstpartition and the centrifugal rotor. The seal may be a gap sealing theform of a narrow passage. Thus the gas stream is forced into theseparation passages when flowing from the first section to the secondsection.

The vessel may be configured to permit a pressure of at least 10 bars inthe gas stream guided by the vessel. The centrifugal separator maycomprise a portion of the vessel. In such a vessel the centrifugalseparator is particularly beneficial since the separator may be operatedinside the vessel without any major modifications to the vessel, such aselectrical or mechanical components or connectors led through the vesselwall.

The frame may be a self-supporting frame for mounting inside an existingvessel for guiding the gas stream, and wherein the frame comprising aholding means to hold the frame at a position inside the vessel. Thusthe centrifugal separator may be fitted in existing vessel systems, suchas pipelines for transporting gas or air ducts and the like. This isbeneficial since the vessel system does not need to be reconstructed formounting the centrifugal separator, and the vessel system may bemaintained and optimized to withstand high pressures and/or theinstallation may be simplified. The frame may be configured to bereleasably mountable in an existing vessel for guiding the gas stream.

The frame may comprise a passage upstream of the rotor, wherein thevortex generator configured to bring the gas stream in rotation isarranged in the passage. A flow directing element may be providedupstream of the passage to direct the gas stream into the passage. Thusthe flow of the gas stream may be efficiently forced into the vortexgenerator configured to bring the gas stream in rotation.

The plurality of separation plates may comprise a stack of frustoconicalseparation discs provided at mutual distances from one another, definingthe separation passages between the discs and wherein each separationdisc is provided with distance members extending from a radially innerportion of the separation disc to a radially outer portion of theseparation disc to define the separation passages between the discs ofthe stack of frustoconical separation discs. Thus the rotation of thegas stream may efficiently be transferred to a rotation of the rotorupon spinning up of the rotating gas in the separation passages. Thedistance members may increase the efficiency by providing a function asvanes transferring rotational momentum from the gas to the rotor. Thedistance members may alternatively or additionally comprise distancemembers in the form of dot-shaped caulks or microcaulks, distributedover the surface of the separation discs.

The separation plates may be formed in polymeric material or in metal,such as stainless steel.

The centrifugal separator may be configured to provide the rotor with arotational speed in the range of 100-11000 rpm, preferably 1000-3000rpm, during operation of the device and driven by the rotational flow ofthe gas stream. The separation is efficient even at relatively lowrotational speeds.

The invention further relates to a method of separating particles from agas stream, the method comprising providing a centrifugal rotor arrangedto be rotatable around a rotational axis and comprising a plurality ofseparation plates defining separation passages between the plates, acentral gas chamber in the rotor communicating with a radially innerportion of the separation passages and the gas outlet, a spacesurrounding the rotor and communicating with a radially outer portion ofthe separation passages and the gas inlet, bringing a gas stream inrotation upstream of the rotor, and rotating the rotor by the rotationalflow of the gas stream for separating particles from the gas stream.

The gas stream may be a stream of fossil gas, natural gas, biogas,exhaust gas, ventilation gas, crankcase gas, carbon dioxide (CO₂),hydrogen sulfide (H₂S), etc.

The invention further relates to the use of a centrifugal separator asdisclosed for separation of particles, such as solid or liquid particlesfrom a stream of gas, such as a stream of fossil gas, natural gas,biogas, exhaust gas, ventilation gas, crankcase gas, carbon dioxide(CO₂), hydrogen sulfide (H₂S), etc, and/or applied to positions in gascompression, amine processes, Shell Claus off-gas treating (SCOT)processes, in exhaust gas scrubbing and the like.

The invention further relates to the use of a centrifugal separator asdisclosed for separation of particles, such as solid or liquid particlesfrom a stream of gas in a pressure vessel, which vessel may beconfigured to permit a pressure of at least 10 bars in the gas streamguided by the vessel.

BRIEF DESCRIPTION OF DRAWINGS

The invention is now described, by way of example, with reference to theaccompanying drawings, in which:

FIG. 1 shows a cross-section along the rotational axis of a centrifugalseparator according to the invention, arranged in a cylindrical vesselfor conveying a gas stream.

FIG. 2 shows a partially cut-out perspective view of a rotor of such acentrifugal separator.

FIG. 3 shows an axial cross-section of a holding means withfrustoconical slotted disc.

FIG. 4 shows a perspective view of a vortex generator configured tobring the gas stream in rotation.

FIG. 5 shows a cross-section perpendicular to the rotational axis of avortex generator configured to bring the gas stream in rotationaccording to another embodiment.

DESCRIPTION OF EMBODIMENTS

In FIG. 1 a centrifugal separator 1 for separation of particles from agas stream is shown arranged in a cylindrical vessel 19 in the form of acylindrical pipe for guiding the gas stream. The separator comprises aself-supporting frame 2 for mounting inside the vessel 19.Self-supporting is understood as an ability of the frame to supportitself without relying on support from the vessel such as duringmounting and dismounting. The frame is provided with a first partition15 for dividing the vessel into a first section 16 upstream of thepartition and a second section 17 downstream of the partition. Theseparator further comprises a gas inlet 3 communicating with the firstsection and a gas outlet 4 communicating with the second section.

The centrifugal separator further comprises a centrifugal rotor 5arranged to be rotatable in the frame around a rotational axis x. Therotational axis extends in the direction of the extension of the vessel.The rotor comprises a shaft 26 having a first and a second end portion.The first end portion is supported in a first frame portion 15 a bymeans of a first bearing 13. The first frame portion 15 a comprises thefirst partition 15. The second end portion is supported in the frame bymeans of a second bearing 14 held in a second frame portion 21. Withreference to FIG. 2, the rotor is described in more detail. The rotorcomprises a disc support structure 27 connected to the rotor axis andextending between the first and second end portions of the rotor axis.The disc support structure has three plate like wings extending alongthe rotor axis and radially outwards from the rotor axis. In analternative embodiment the disc support structure comprises two or morewings, such as six wings. Towards the second end portion of the rotoraxis, a bottom disc 28 is attached to the wings of the disc supportstructure. On the bottom disc, and guided by the radially outer portionsof the plate like wings, a plurality of frustoconical separation discs 6are stacked. The separation discs may be made of a lightweight materialsuch as plastic, or of metal such as stainless steel. The separationdiscs are each provided with distance members in order to provideseparation passages 7 between the discs in the stack. The distancemembers are in the form of elongated protrusions extending from aradially inner portion to a radially outer portion of each separationdisc, having an extension along a line or a curve. The elongateddistance members, or caulks, may be straight or curved and may beintegrated in the discs or attached to the discs. The distance membersmay alternatively or additionally comprise distance members in the formof dot-shaped caulks or microcaulks, distributed over the surface of theseparation discs. On top of the stack of separation discs a top disc 29is provided. The top disc is attached to the wings of the disc supportstructure. The stack of separation discs are compressed by the top discand the bottom disc. Radially inside the separation discs a central gasspace 8 is formed, divided into three parts by the wings of the discsupport structure 27. The top disc is provided with a central opening 30such that the central gas space of the rotor is open for passage of gasthrough the top disc. The top disc is provided with a flange 31circumventing the central opening providing a cylindrical outer sealingsurface, 18 a.

Again turning to FIG. 1, a narrow gap is formed between a sealingsurface 18 a formed on the flange 31 of the top disc and a correspondingcylindrical sealing surface 18 b on the first partition. The gap forms agap sealing 18 between the first 16 and second 17 sections in thevessel. The central gas chamber 8 in the rotor communicates with aradially inner portion of the separation passages 7 and the gas outlet 4via the central opening of the top disc and openings 32 formed in thefirst partition, surrounding the first bearing 13. Further, a firstspace 9 is formed between the inner surface of the frame 2 and radiallyoutside and surrounding the rotor. The space 9 surrounding the rotorcommunicates with the radially outer portion of the separation passages7 and a second space 41 formed by the inner surface of the frame 2 andin the axial space between the vortex generator and separation plates.Both the first space and second space are formed in the flow pathbetween the vortex generator and separation plates. The centrifugalseparator is configured such that the first and second sections of thevessel communicate via the separation passages 7 of the rotor.

The frame comprises a bottom sealing ring 33 forming the gas inlet 3 inthe frame. The bottom sealing ring is sealingly connected, 38, to theinner vessel wall 25. A cylindrical frame tube 24 extends along theinner wall of the vessel as a part of the frame, from the bottom sealingring to the first partition 15 and connects with the other parts of theframe to provide a self-supporting frame structure. The second frameportion 21 supporting the second bearing 14 is connected to andsupported by the inner wall of the cylindrical frame tube. The spacebetween the gas inlet 3 and device 10, discussed below, creates achamber 22.

The frame 2 further comprises a holding means 20 to hold the frame at aposition inside the vessel. The holding means comprises in a ring shapedpart 34 sealingly connected, by means of a sealing member 37, to theinner vessel wall 25. The holding means is configured to engage with thecylindrical inner surface of the vessel by providing an expandable outerdiameter. With reference to FIG. 3, the holding means is described inmore detail. The ring shaped part 34 is connected to the first partition15 by a plurality of bolts 35 distributed around the circumference ofthe ring shaped part 34. The holding means comprises one or moreradially slotted frustoconical discs 36 mounted such that compression ofthe disc by tightening the bolts of the ring shaped part causes slottedradially outer portions 36 a of the disc to expand and engage with thecylindrical inner surface of the vessel. Thus the expandable outerdiameter is realized by a by tightening the compressive bolts 35.

Again with reference to FIG. 1, the centrifugal separator comprises astationary vortex generator 10 configured to bring the gas stream inrotation. The vortex generator 10 configured to bring the gas stream inrotation is positioned upstream of the rotor and formed in the secondframe portion 21. The vortex generator 10 comprises a gas deflectingmember 11 comprising a plurality of vanes 12 which are inclined withrespect to the axial direction x of the centrifugal rotor anddistributed around the rotational axis. The vanes are arranged in apassage 11 a formed in the second frame portion upstream of the rotor.The passage 11 a extends radially outside the separation plates of thecentrifugal rotor. With reference to FIG. 4, the vortex generator 10configured to bring the gas stream in rotation is shown in furtherdetail. The vortex generator 10 comprises a gas deflecting ring 11comprising a plurality of vanes 12 extending outwardly from the gasdeflecting ring and distributed around the rotational axis of the rotor.The vanes are inclined with respect to the axial direction of the rotorso that a first end of each vane is spaced from a second end of the vanein the axial and circumferential direction, which inclination isgradually increased along the length of the vanes in the direction ofthe flowing gas so that the vanes are curved, as seen in FIG. 4.

According to one embodiment, the vanes may be movable/or and theinclination of the vanes may be adjusted during operation in order tocontrol the speed of rotation of the gas stream.

In addition to, or as an alternative to what is shown in FIG. 4, the gasinlet 3 upstream of the centrifugal rotor may be arranged at a rightangle to the rotational axis of the centrifugal rotor, as shown in FIG.5. This figure shows a cross-section of the vessel, perpendicular to therotational axis of the rotor, at the inlet side of the centrifugalseparator. It is preferred to connect external pipe connections at rightangles in order to withstand high pressure in the vessel. In thisembodiment, the vortex generator 10 configured to bring the gas streamin rotation upstream of the rotor comprises an inlet gas deflectingmember 11′ which is arranged to deflect the gas stream from the gasinlet towards a tangential direction of the centrifugal rotor. The inletgas deflecting member 11′ may be stationary or pivotally connected tothe vessel 19 and may be slanted or bent such that gas flowing throughthe inlet 3 is deflected towards a tangential direction of thecentrifugal rotor, thus achieving a rotational flow of the gas stream inthe vessel. The position or inclination of the gas deflecting ring maybe adjusted during operation of the separator such that to control therotational speed of the gas stream. As shown, this may be achieved bythe inlet gas deflecting member 11′ being pivotally connected to thevessel at a point 39, and biased towards an initial position by means ofa spring 40. The spring may be integrated with the inlet gas deflectingmember at the pivot point or connecting the inlet gas deflecting memberto another point of the vessel. At an increasing flow of gas the inletgas deflecting member is deflected by the gas flow, which may result ina limitation of the speed of rotation of the gas in the vessel.

With reference to FIG. 1, the separator is mounted in the vessel 19 byplacing the separator with its self-supporting frame 2 inside thevessel, at a desired position inside the vessel, and expanding thediameter of the holding means 20 so that the holding means engage withthe inner surface 25 of the vessel, to hold the separator at the desiredposition inside the vessel.

During operation of the centrifugal separator a stream of gas entersinto the inlet 3 of the centrifugal separator 1. The stream of gas isforced into the passage 11 a where the inclined vanes 12 deflect the gastowards a tangential direction of the rotor of the separator. Thus thegas stream is brought into rotation by the vanes 12, and enters into thespace 9 surrounding the rotor 5. In this space a pre-separation occurswhereas larger particles in the form of solid particles and/or liquiddroplets having a density larger than the gas in the gas stream areseparated from the gas stream by means of centrifugal forces in therotating gas stream and deposited on the inner surface of the cylinder24.

From the space 9 surrounding the rotor, the rotating gas stream entersinto the separation passages 7 formed between the separation discs 6 inthe rotor. The rotor 5 is brought into rotation by the rotating gasstream by means of viscous forces acting on the separation discs in theseparation passages. The rotation of the rotor is also facilitated bythe elongated distance members of the disc stack working as vanes orturbine blades to improve the transfer of momentum from the gas streamto the rotor. Since the rotating gas stream is led from the radiallyouter portions of the separation passages and towards the radially innerportions of the separation passages, the gas stream is spun up thanks tothe conservation of angular momentum. Thus the transfer of the rotationfrom the gas to the rotor is particularly efficient in thisconfiguration.

In the separation passages, particles in the form of solid particlesand/or liquid droplets having a density larger than the gas in the gasstream are separated from the gas stream by centrifugal forces. Due tothe smaller separation distances in the separation passages of the stackof frustoconical discs this even allows for separation of smaller and/orless dense particles from the gas stream. Particles separated from thegas stream are deposited on the inner surface of the frustoconicalseparation discs and transported radially outwardly by means ofcentrifugal forces. From the radially outer edge of the separationdiscs, particles separated from the gas stream in the separationpassages are thrown towards and deposited at the inner surface of thecylinder 24.

Thus the rotational flow of the gas mixture alone drives the rotation ofthe centrifugal rotor, without a drive motor driving the rotor. Theresulting rotation causes separation of particles from the same gasstream. Cleaned gas conducted towards the central gas chamber 8 of therotor is provided to the outlet 4 through the passages 30 and 32 formedin the rotor and the first partition, and transported from the separatorthrough the vessel.

The invention claimed is:
 1. A centrifugal separator for separation ofparticles from a gas stream, comprising: a frame, a gas inlet and a gasoutlet; a centrifugal rotor arranged to be rotatable in the frame arounda rotational axis and comprising a plurality of separation platesstacked along an axial length of the rotor and defining separationpassages between the plates; a central gas chamber in the rotorcommunicating with a radially inner portion of the separation passagesand the gas outlet; a first space surrounding the rotor andcommunicating with a radially outer portion of the separation passagesand the gas inlet; and a vortex generator axially spaced from thecentrifugal rotor to create a second space in an axial space between thevortex generator and the separation plates to bring the gas stream inrotation upstream of the plurality of separation plates and downstreamof the gas inlet, the first space and second space defining a flow pathbetween the vortex generator and separation plates such that the gasstream flows from the vortex generator and then radially inward towardthe separation plates, and the vortex generator comprising a passagehaving an entrance and an exit, the exit being downstream of theentrance, wherein a vortex created by the vortex generator enters thefirst space and drives the rotation of the centrifugal rotor forseparating particles from the same gas stream by being conducted fromthe first space surrounding the rotor, through the separation passagesbetween the plates and towards the central gas chamber, and wherein thevortex generator configured to bring the gas stream in rotation isnon-rotating during operation of the apparatus.
 2. The centrifugalseparator according to claim 1, wherein the vortex generator isconfigured to bring the gas stream in rotation by deflecting the gasstream towards a tangential direction of the centrifugal rotor.
 3. Thecentrifugal separator according to claim 2, wherein the vortex generatorcomprises a gas deflecting ring having at least one vane inclined withrespect to the axial direction of the centrifugal rotor.
 4. Thecentrifugal separator according to claim 3, wherein the at least onevane extends radially outside the separation plates of the rotor.
 5. Thecentrifugal separator according to claim 4, wherein there are aplurality of vanes and the inclination of the vanes with respect to theaxial direction of the centrifugal rotor increases gradually along theextent of the vanes in the direction of the flow of the gas stream fromthe gas inlet to the gas outlet.
 6. The centrifugal separator accordingto claim 3, wherein there are a plurality of vanes distributed aroundthe rotational axis and the inclination of the vanes with respect to theaxial direction of the centrifugal rotor increases gradually along theextent of the vanes in the direction of the flow of the gas stream fromthe gas inlet to the gas outlet.
 7. The centrifugal separator accordingto claim 1, wherein the gas inlet is arranged at an angle to therotational axis of the centrifugal rotor, wherein the angle is withinthe range of 70-110 degrees, and wherein the vortex generator configuredto bring the gas stream in rotation upstream of the rotor comprises aninlet gas deflecting member arranged to deflect the gas stream from thegas inlet towards a tangential direction of the centrifugal rotor. 8.The centrifugal separator according to claim 1, wherein the rotor has afirst and a second axial end portion, and wherein the rotor is rotatablysupported in the frame by means of a first bearing at the first axialend portion and a second bearing at the second axial end portion.
 9. Thecentrifugal separator according to claim 1, wherein the gas inlet isarranged at an angle to the rotational axis of the centrifugal rotor,wherein the angle is within the range of 80-100 degrees, and wherein thevortex generator configured to bring the gas stream in rotation upstreamof the rotor comprises an inlet gas deflecting member arranged todeflect the gas stream from the gas inlet towards a tangential directionof the centrifugal rotor.
 10. The centrifugal separator according toclaim 1, wherein the gas inlet is arranged at an angle of 90 degrees tothe rotational axis of the centrifugal rotor, and wherein the vortexgenerator configured to bring the gas stream in rotation upstream of therotor comprises an inlet gas deflecting member arranged to deflect thegas stream from the gas inlet towards a tangential direction of thecentrifugal rotor.
 11. The centrifugal separator according to claim 1,wherein the vortex generator extends across an entire width of theframe.
 12. The centrifugal separator according to claim 1, wherein thevortex generator comprises a ring shaped gas deflecting member and aplurality of inclined vanes extending outwardly from the ring shaped gasdeflecting member a first end of each vane spaced from a second end ofeach vane in the axial and circumferential direction.
 13. Thecentrifugal separator according to claim 1, wherein the outlet extendsalong a longitudinal axis of the central gas chamber.
 14. A centrifugalseparator for separation of particles from a gas stream, comprising: aframe, a gas inlet and a gas outlet; a centrifugal rotor arranged to berotatable in the frame around a rotational axis and comprising aplurality of separation plates defining separation passages between theplates; a central gas chamber in the rotor communicating with a radiallyinner portion of the separation passages and the gas outlet; a firstspace surrounding the rotor and communicating with a radially outerportion of the separation passages and the gas inlet; and a vortexgenerator axially spaced from the centrifugal rotor to create a secondspace in an axial space between the vortex generator and the separationplates, the vortex generator configured to bring the gas stream inrotation upstream of the rotor and downstream of the gas inlet, thefirst space and second space defining a flow path between the vortexgenerator and separation plates such that the gas stream flows from thevortex generator and then radially inward toward the separation plates,and the vortex generator comprising a passage having an entrance and anexit, the exit being downstream of the entrance, wherein a vortexcreated by the vortex generator enters the first space and drives therotation of the centrifugal rotor for separating particles from the samegas stream by being conducted from the first space surrounding therotor, through the separation passages between the plates and towardsthe central gas chamber, wherein the vortex generator configured tobring the gas stream in rotation is non-rotating during operation of theapparatus, wherein the frame is configured to be mountable inside avessel for guiding the gas stream, and comprises a first partition fordividing the vessel into a first section upstream of the first partitionand a second section downstream of the first partition, wherein the gasinlet is in communication with the first section, the gas outlet is incommunication with the second section, and wherein the centrifugalseparator is configured such that the first and second sectionscommunicate via the separation passages of the rotor.
 15. Thecentrifugal separator according to claim 14, wherein a seal is providedbetween the first partition and the centrifugal rotor.
 16. Thecentrifugal separator according to claim 14, wherein the frame is aself-supporting frame for mounting inside an existing vessel for guidingthe gas stream, and wherein the frame comprises a holder to hold theframe at a position inside the vessel.
 17. The centrifugal separatoraccording to claim 14, wherein the frame comprises a passage upstream ofthe rotor, and wherein the vortex generator configured to bring the gasstream in rotation is arranged in the passage.
 18. A method ofseparating particles from a gas stream, the method comprising the stepsof: providing a centrifugal rotor arranged to be rotatable around arotational axis and comprising a plurality of separation plates stackedalong an axial length of the rotor and defining separation passagesbetween the plates, a central gas chamber in the rotor communicatingwith a radially inner portion of the separation passages and a gasoutlet, and a first space surrounding the rotor and communicating with aradially outer portion of the separation passages and a gas inlet;deflecting a gas stream by bringing a gas stream moving in an axialdirection in rotation upstream of the plurality of separation plates anddownstream of the gas inlet with a vortex generator spaced from thecentrifugal rotor to create a second space between the vortex generatorand the separation plates; creating a vortex in the first space;rotating the rotor by the rotational flow of the gas stream contactingthe plurality of separation plates for separating particles from the gasstream by conducted the vortex radially inwardly from the first spacesurrounding the rotor, through the separation passages between theplates and towards the central gas chamber.